Surface treatment of TiO2 films for dye-sensitized solar cells using atmospheric-pressure non-equilibrium DC pulse discharge plasma jet

In this paper, we report the utilization of the DC pulse discharge plasma jet technique as a means for the preparation of titanium oxide (TiO₂) films on fluorine dope tin oxide (FTO) coated glass substrates used for dye-sensitized solar cells (DSCs). The TiO₂ film made on these experimental bases exhibited the BET specific surface area of 95 m²/g, the pore volume of 0.3 cm²/g and the TEM particle size of ∼25 nm. The DSCs made by the TiO₂ film exhibited an energy conversion efficiency of 5.7% at 100 mW/cm² light intensity. Consequently, we believe that the optimization between the specific surface area and photocurrent density of TiO₂ film was achieved by the plasma surface treatment which also contributed to the improvement of energy conversion efficiency of DSCs.     

Preparation of anatase TiO2 thin films by vacuum arc plasma evaporation

Anatase titanium dioxide (TiO₂) thin films with high photocatalytic activity have been prepared with deposition rates as high as 16 nm/min by a newly developed vacuum arc plasma evaporation (VAPE) method using sintered TiO₂ pellets as the source material. Highly transparent TiO₂ thin films prepared at substrate temperatures from room temperature to 400 °C exhibited photocatalytic activity, regardless whether oxygen (O₂) gas was introduced during the VAPE deposition. The highest photocatalytic activity and photo-induced hydrophilicity were obtained in anatase TiO₂ thin films prepared at 300 °C, which correlated to the best crystallinity of the films, as evidenced from X-ray diffraction. In addition, a transparent and conductive anatase TiO₂ thin film with a resistivity of 2.6 × 10^− 1 Ω cm was prepared at a substrate temperature of 400 °C without the introduction of O₂ gas.     

Influence of annealing treatments for ZnO-doped Zr0.8Sn0.2TiO4 thin films by RF magnetron sputtering

Zirconium tin titanium oxide doped 1 wt.% ZnO thin films on n-type Si substrate were deposited by rf magnetron sputtering at a fixed rf power of 300 W, a substrate temperature of 450 °C, a deposition pressure of 5 mTorr and an Ar/O₂ ratio of 100/0 with various annealing temperatures and annealing times. Electrical properties and microstructures of 1 wt.% ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by rf magnetron sputtering on n-type Si(100) substrates at different annealing temperatures (500 °C-700 °C) and annealing times (2 h-6 h) have been investigated. The structural and morphological characteristics analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM) were sensitive to the treatment conditions such as annealing temperature and annealing time. At an annealing temperature of 600 °C and an annealing time of 6 h, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films possess a dielectric constant of 46 (at f = 10 MHz), a dissipation factor of 0.059 (at f = 10 MHz), and a low leakage current density of 3.8 × 10^− 9 A/cm² at an electrical field of 1 kV/cm.     

RF diode reactive sputtering of n- and p-type zinc oxide thin films

We present the relationship between parameters of reactive RF diode sputtering from a zinc oxide (ZnO) target and the crystalline, electrical and optical properties of n-/p-type ZnO thin films. The properties of the ZnO thin films depended on RF power, substrate temperature and, particularly, on working gas mixtures of Ar/O₂ and of Ar/N₂. Sputtering in Ar+O₂ working gas (up to 75% of O₂) improved the structure of an n-type ZnO thin film, from fibrous ZnO grains to columnar crystallites, both preferentially oriented along the c-axis normally to the substrate (〈0 0 2〉 direction). These films had good piezoelectric properties but also high resistivity (ρ≈10³ Ω cm). ZnO:N p-type films exhibited nanograin structure with preferential 〈0 0 2〉 orientation at 25% N₂ and 〈1 0 0〉 orientation for higher N₂ content. The presence of nitrogen N_O at O-sites forming N_O-O acceptor complexes in ZnO was proven by SIMS and Raman spectroscopy. A minimum value of resistivity of 790 Ω cm, a p-type carrier concentration of 3.6×10¹⁴ cm⁻³ and a Hall mobility of 22 cm² V⁻¹ s⁻¹ were obtained at 75% N₂.     

Characterization of SiO2 and TiO2 films prepared using rf magnetron sputtering and their application to anti-reflection coating

Preparation of TiO₂ and SiO₂ films for optical applications was attempted using conventional rf magnetron sputtering in the sputtering ambient with various O₂/Ar+O₂ ratios and at substrate temperatures between room temperature and 400 °C. X-ray photoelectron spectroscopy (XPS) and optical spectroscopy investigations indicated that oxygen addition in the sputtering ambient was essential for growing TiO₂ films with stoichiometric compositions and good transmittance, while SiO₂ films had a stoichiometric composition of O/Si ratio=2.1-2.2 and were highly transparent in the visible wavelength region, independent of gas composition in the growing ambient. It was also identified from scanning electron microscope (SEM), atomic force microscope (AFM) and Fourier transform infrared spectroscopy (FTIR) measurements that the structural characteristics of both TiO₂ and SiO₂ films were significantly improved with O₂ addition in the sputtering ambient, showing smoother surface morphologies and higher resistances to water absorption when compared with films grown without O₂ addition. Heating of the substrate between 200 and 400 °C considerably increased the refractive index of TiO₂ layers, resulting in dense structures along with an improvement of crystallinity. For optical applications, AR coatings composed of 2-4 multi-layers on glass were designed and manufactured by stacking in turn the SiO₂ and TiO₂ films at room temperature and O₂/Ar+O₂=10%, and the performance of the produced coatings was compared with simulation results.     

The effect of nitrogen on the properties of zinc nitride thin films and their conversion into p-ZnO:N films

Zinc nitride thin films were deposited by magnetron sputtering using ZnN target in plasma containing either N₂ or Ar gases. The rf-power was 100 W and the pressure was 5 mTorr. The properties of the films were examined with thermal treatments up to 550 °C in N₂ and O₂ environments. Films deposited in Ar plasma were opaque and conductive (ρ ∼ 10^− 1 to 10^− 2 Ω cm, N_D ∼ 10¹⁸ to 10²⁰ cm^− 3) due to excess of Zn in the structure. After annealing at 400 °C, the films became more stoichiometric, Zn₃N₂, and transparent, but further annealing up to 550 °C deteriorated the electrical properties. Films deposited in N₂ plasma were transparent but very resistive even after annealing. Both types of films were converted into p-type ZnO upon oxidation at 400 °C. All thermally treated zinc nitride films exhibited a shoulder in transmittance at around 345 nm which was more profound for the Ar-deposited films and particularly for the oxidized films. Zinc nitride has been found to be a wide band gap material which makes it a potential candidate for transparent optoelectronic devices.     

Structural and electrical properties of sputtered indium-zinc oxide thin films

In this study, indium-zinc oxide (IZO) thin films have been prepared at a room temperature, 200 and 300 °C by radio frequency magnetron sputtering from a In₂O₃-12 wt.% ZnO sintered ceramic target, and their dependence of electrical and structural properties on the oxygen content in sputter gas, the substrate temperature and the post-heat treatment was investigated. X-ray diffraction measurements showed that amorphous IZO films were formed at room temperature (RT) regardless of oxygen content in sputter gas, and micro-crystalline and In₂O₃-oriented crystalline films were obtained at 200 and 300 °C, respectively. From the analysis on the electrical and the structural properties of annealed IZO films under Ar atmosphere at 200, 300, 400 and 500 °C, it was shown that oxygen content in sputter gas is a critical parameter that determines the local structure of amorphous IZO film, stability of amorphous phase as well as its eventual crystalline structure, which again decide the electrical properties of the IZO films. As-prepared amorphous IZO film deposited at RT gave specific resistivity as low as 4.48 × 10^− 4 Ω cm, and the highest mobility value amounting to 47 cm²/V s was obtained from amorphous IZO film which was deposited in 0.5% oxygen content in sputter gas and subsequently annealed at 400 °C in Ar atmosphere.     

Effect of sputtering pressure and post-annealing on hydrophilicity of TiO2 thin films deposited by reactive magnetron sputtering

A series of TiO₂ thin films was deposited onto glass substrates without intentional heating or biasing by magnetron sputtering of a titanium target using Ar/O₂ reactive mixtures over a broad range of total sputtering pressures from 0.12 Pa to 2.24 Pa. Each of the film types was deposited by the threshold poisoned mode at a specific given oxygen flow rate monitored in-situ by optical emission spectroscopy. Both the sputtering pressure and thermal annealing are the key factors for the TiO₂ films to yield fast-response superhydrophilicity with a water contact angle of 5°. The mechanism of superhydrophilicity for the TiO₂ films deposited by high-pressure sputtering will be discussed based on empirical studies of X-ray diffractometry, high-resolution scanning microscopy and atomic force spectroscopy.     

Fabrication of high frequency ZnO thin film SAW devices on silicon substrate with a diamond-like carbon buffer layer using RF magnetron sputtering

Diamond-like carbon (DLC) film is a promising candidate for surface acoustic wave (SAW) device applications because of its higher acoustic velocity. A zinc oxide (ZnO) thin film has been deposited on DLC film/Si substrate by RF magnetron sputtering; the optimized parameters for the ZnO sputtering are RF power density of 0.55 W/cm², substrate temperature of 380 °C, gas flow ratio (Ar/O₂) of 5/1 and total sputter pressure of 1.33 Pa. The results showed that when the thickness of the ZnO thin films was decreased, the phase velocity of the SAW devices increased significantly.     

Solar water oxidation in sputter-deposited nanocrystalline WO3 photoanodes via tuning of Ar:O2 flow rate combinations

Thin film WO₃ photoanodes were prepared by reactive sputtering in Ar and O₂ gas mixtures of various flow rate combinations. Furnace annealed films were nanocrystalline monoclinic WO₃ with (002), (020) and (200) plane orientations. Water oxidation in 0.33 M H₂SO₄ electrolyte under simulated solar illumination showed that photoanodes deposited in highest Ar and O₂ flow rate combinations exhibited highest photocurrent of 4.1 mA cm⁻² (at 1.3 V vs Ag/AgCl) compared to 3–3.8 mA cm⁻² for photoanodes deposited in lower flow rate combinations. The higher photocurrents were ascribed to lower bulk resistivity and charge transfer resistance at the WO₃/electrolyte interface. These photoanodes consisted of randomly oriented (002), (020) and (200) planes in contrast to the preferentially orientated (002) and (200) planes of photoanodes which were highly resistive with poorer photocurrent responses. These results were interpreted in terms of the effects of Ar:O₂ flow rate combinations on the distribution of oxygen vacancies and formation of crystallographic shear planes in the sputtered films.     

Characterization of gallium-nitrogen co-doped zinc oxide thin films prepared by RF diode sputtering

We investigated the possibility of achieving p-type zinc oxide (ZnO) by RF diode sputtering and gallium-nitrogen co-doping. ZnO:Ga:N thin films were prepared with a different N₂ content in Ar/N₂ working gas, ranging from 0 to 100%, and at a varying substrate temperature, from room temperature (RT) to 300 °C. A hole conduction with maximum carrier concentration of 2.6 × 10¹⁸ cm⁻³, mobility of 2 cm²/Vs and resistivity of 1.5 Ω cm resulted from deposition at RT with 100% N₂. It arose from N incorporation and formation of N_O acceptors. In the secondary ion mass spectrometry (SIMS) depth profiles of the co-doped films were observed NO/NO₂ negative ions. Average transmittance (including Corning glass substrate) across the visible spectrum varied (60 ÷ 66%) with increasing nitrogen content (50 ÷ 100% N₂). As the substrate temperature increased (RT - 300 °C), highly transparent (T ∼72-83%) and conductive (electron concentrations of 10¹⁷-10¹⁹ cm⁻³) n-type ZnO:Ga:N films were attained. Reduction of optical band gap (E_g) (∼3.13-3.08 eV) was observed for co-doped ZnO films. Atomic force microscopy (AFM) images revealed that the films grown at RT have roughness of approximately 5.3 nm while roughness of those grown at 300 °C is approximately 3.9 nm.     

Preparation and characterization of La0.9Sr0.1Ga0.8Mg0.2O3−δ thin film electrolyte deposited by RF magnetron sputtering on the porous anode support for IT-SOFC

Thin films of solid electrolyte La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) were deposited by RF magnetron sputtering onto porous La_0.7Sr_0.3Cr_0.5Mn_0.5O_3−δ (LSCM) anode substrates. The effects of substrate temperature, sputtering power density and sputtering Ar gas pressure on the LSGM thin film density, flatness and morphology were systematically investigated. RF sputtering power density of 7.8 W cm⁻², substrate temperature of 300 °C and sputtering Ar gas pressure of 5 Pa are identified as the best technical parameters. In addition, a three-electrode half cell configuration was selected to investigate the electrochemical performance of the thin film. The LSGM film deposited at optimum conditions exhibited a lower area specific ohmic resistance of 0.68 Ω cm⁻² at 800 °C, showing that the practicability of RF magnetron sputtering method to fabricate LSGM electrolyte thin film on porous LSCM anode substrates.     

Room temperature crystallization by RF plasma

The crystallization of amorphous thin films was achieved by radiofrequency (RF) plasma treatment. Although various amorphous films are crystallized after 2 min or so, the sample temperature is lower than 150 °C without compulsory cooling even when the films are treated for 1 h. This treatment works on amorphous films of various materials, independently of the film preparation method and substrate materials. Sol-gel-derived TiO₂ films were densified and simultaneously crystallized to anatase structure by the plasma treatment and the obtained films indicate almost the same photocatalytic activities as that of thermally crystallized TiO₂ films. Plasma-crystallized sputtered indium tin oxide (ITO) films have a bixbite structure and the resistivity reached to 1.6 × 10^− 4 Ω cm while the crystallization condition was not optimized. Amorphous silicon films with a small mount of crystallites were deposited by sputtering method and were crystallized by the plasma treatment.     

The effect of sputtering gas pressure on structure and photocatalytic properties of nanostructured titanium oxide self-cleaning thin film

Titanium oxide thin films were deposited by DC reactive magnetron sputtering on ZnO (80 nm thickness)/soda-lime glass and SiO₂ substrates at different gas pressures. The post annealing on the deposited films was performed at 400 °C in air atmosphere. The results of X-ray diffraction (XRD) showed that the films had anatase phase after annealing at 400 °C. The structure and morphology of deposited layers were evaluated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface grain size and roughness of TiO₂ thin films after annealing were around 10-15 nm and 2-8 nm, respectively. The optical transmittance of the films was measured using ultraviolet-visible light (UV-vis) spectrophotometer and photocatalytic activities of the samples were evaluated by the degradation of Methylene Blue (MB) dye. Using ZnO thin film as buffer layer, the photocatalytic properties of TiO₂ films were improved.     

High-rate deposition of Ta-doped SnO2 films by reactive magnetron sputtering using a Sn-Ta metal-sintered target

Ta-doped SnO₂ films were deposited on glass substrate (either unheated or heated at 200 °C) by reactive magnetron sputtering with a Sn-Ta metal-sintered target using a plasma control unit (PCU) and mid-frequency (mf, 50 kHz) unipolar pulsing. The PCU feedback system precisely controlled the flow of the reactive and sputtering gases (O₂ and Ar, respectively) by monitoring either discharge impedance or the plasma emission of the atomic O* line at 777 nm. The planar target was connected to the switching unit, which was operated in unipolar pulse mode. Power density on the target was maintained at 4.4 W cm^− 2 during deposition. The lowest obtained resistivity for the films deposited on heated substrate was 6.4 × 10^− 3 Ωcm, where the deposition rate was 250 nm min^− 1.     

Low temperature preparation of transparent, antireflective TiO2 films deposited at different O2/Ar ratios by microwave electron cyclotron resonance magnetron sputtering

A serial of crystalline titanium oxide ceramic films were deposited at low temperature using microwave electron cyclotron resonance (MW-ECR) magnetron sputtering with different O₂/Ar ratios. The influences of O₂/Ar ratio on the deposition rate, morphology, crystalline nature, optical adsorption property of the obtained titanium oxide thin films were investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and UV-Vis spectra. Therefore, the optimum O₂/Ar ratio for deposition of anatase TiO₂ thin films on unheated glass substrate was realized in a MW-ECR magnetron sputtering process. The as-deposited anatase TiO₂ films were transparent and were antireflective in the visible region.     

Preparation of cobalt oxide films by plasma-enhanced metalorganic chemical vapour deposition

Co oxide films were prepared on glass substrates at 150–400°C by plasma-enhanced metalorganic chemical vapour deposition using cobalt (II) acetylacetonate as a source material. NaCl-type CoO films were formed at low O₂ flow rate of 7cm³ min^–1 and at a substrate temperature of 150–400°C. The CoO films possessed (100) orientation, independent of substrate temperature. Deposition rates of the CoO films were 40–47 nm min^–1. The CoO film deposited at 400 °C was composed of closely packed columnar grains and average diameter size at film surface was 60 nm. At high O₂ flow rate of 20–50 cm³ min^–1, high crystalline spinel-type Co₃O₄ films were formed at a substrate temperature of 150–400°C. The Co₃O₄ film deposited at 400°C possessed (100) preferred orientation and the film deposited at 150°C possessed (111) preferred orientation. Deposition rates of the Co₃O₄ films were 20–41 nm min^–1. Both Co₃O₄ films with (100) and (111) orientation had columnar structure. The shape and average size of the columnar grains at the film surface were different; a square shape and 35 nm for (100)-oriented Co₃O₄ film and a hexagonal shape and 60 nm for (111)-oriented film, respectively.     

Photocatalytic properties of Cr-doped TiO2 films prepared by oxygen cluster ion beam assisted deposition

We prepared Cr-doped titanium dioxide (TiO₂) films by oxygen (O₂) cluster ion beam assisted deposition method, and investigated photocatalytic properties of the films as well as crystallographic property, optical property and surface morphology. The films prepared at a substrate temperature below 200 °C were found to be amorphous from the X-ray diffraction measurement. For the substrate temperatures such as 300 °C and 400 °C, the films exhibited rutile and/or anatase structures. The film surface measured by the atomic force microscope (AFM) was smooth at an atomic level. Furthermore, the optical band gap decreased with increase of Cr-composition, and it was approximately 3.3 eV for the non-doped films, 3.2 eV for the 1% Cr-doped films and 3.1 eV for the 10% Cr-doped films, respectively. With regard to the photocatalytic properties of the Cr-doped TiO₂ films, we measured the change of contact angle as well as the photocatalytic degradation of methylene blue by the UV light irradiation. Compared with the non-doped films, the 1% Cr-doped films prepared at a substrate temperature of 400 °C showed high degradation efficiency. In addition, the contact angle of the 1% Cr-doped films with an initial value of 60° decreased to 10° by the UV light irradiation for 20 min, and the films exhibited the predominant properties of photocatalytic hydrophilicity even for the UV light irradiation with longer wavelengths.     

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular beam epitaxy

The present study reports on the growth of thin TiO₂ films onto Au(100) single crystals by Ti evaporation in a reactive O₂ atmosphere at two different substrate temperatures: room temperature (RT) and 300 °C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively.The TiO₂ film grows epitaxially on the Au(100) substrate at 300 °C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti^3 + ions) at the Au(100) interface. A crystalline and stoichiometric TiO₂ oxide is produced at high substrate temperature, while growth at RT gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O₂ atmosphere restores the sample to the as-grown conditions.     

Optical waveguiding in amorphous tellurium oxide thin films

Optical waveguiding characteristics of amorphous TeO_2−x films deposited by reactive sputtering under different O₂:Ar gas mixtures are investigated on fused quartz and Corning glass substrates. Infra-red absorption band in the range 641-658 cm⁻¹ confirmed the formation of a TeO bond, and a 20:80 O₂:Ar gas mixture ratio is found to be optimum for achieving highly uniform and transparent films at a high deposition rate. As grown amorphous films exhibited a large band gap (3.76 eV); a high refractive index value (2.042-2.052) with low dispersion over a wide wavelength range of 500-2000 nm. Optical waveguiding with low propagation loss of 0.26 dB/cm at 633 nm is observed on films subjected to a post-deposition annealing treatment at 200 °C. Packing density and etch rates have been determined and correlated with the lowering of optical propagation loss in the annealed films.